zhangheff
幼苗
共回答了9个问题采纳率:100% 举报
Cn取m+Cn取m+1=Cn+1取m+1(公式)
则,C10取9 + C11取9 + C12取9 + ...+ C19取9
=C10取10+C10取9 + C11取9 + C12取9 + ...+ C19取9 -C10取10
=C11取10+C11取9 + C12取9 + ...+ C19取9 -1
=C12取10+ C12取9 + ...+ C19取9 -1
=C13取10+C13取10+.+ C19取9 -1
=.=C19取10+ C19取9 -1
=C20取10 -1
1年前
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